1. Field of the Invention
This invention relates to diagnostic systems for automobiles.
2. Prior Art
It is known to use a catalyst, such as a three-way catalyst, in the exhaust system of an automobile to reduce various types of emissions. It is also known to attempt to determine the condition of the catalyst through the use of exhaust gas oxygen sensors, such as, for example, a heated exhaust gas oxygen sensor (the HEGO), positioned downstream of the catalyst. It is also known to use such a HEGO sensor to achieve improved performance of the air fuel feedback control system for the vehicle engine. The prior art has taught that HEGO sensors located upstream and downstream of a catalyst can be used to monitor the condition of the catalyst when artificial A/F modulation signals are applied to the engine (see U.S. Pat. No. 5,077,970). Since the application of such modulating signals is considered intrusive to the operation of the engine, a method for monitoring the catalyst condition by using the characteristics of the normal post-catalyst feedback system without applying any artificial A/F modulation is desired.